1. Field of the Invention
The present invention relates to a method of recovering ammonia by pressurizing and cooling exhaust gas containing ammonia, the exhaust gas being exhausted from a production process of a gallium nitride compound semiconductor and then by liquefying ammonia contained in the exhaust gas to recover the liquefied ammonia from the exhaust gas. The present invention also relates to a method of recycling ammonia, including supplying the recovered ammonia to the production process of a gallium nitride compound semiconductor as a source gas.
2. Related Art
The gallium nitride compound semiconductor is extensively used as a device such as a light emitting diode or a laser diode. A production process of this gallium nitride compound semiconductor (gallium nitride compound semiconductor process) is typically conducted by vapor-depositing a gallium nitride compound on a substrate of sapphire or the like through the MOCVD method. As the source gas used for this production process, for example, a group III source gas including trimethyl gallium, trimethyl indium, and trimethyl aluminum, and a group V source gas including ammonia are used.
Ammonia is required in very large quantities compared with a group III gas source such as trimethyl gallium because of the poor degradation efficiency of ammonia. Ammonia used for the production process of a gallium nitride compound semiconductor is highly pure ammonia generated by distilling or rectifying industrial ammonia or high-priced ammonia generated by further purifying this highly pure ammonia. Most of the ammonia is not used in a production process of a gallium nitride compound semiconductor and wasted in large quantities as unreacted. Therefore, it is desirable that ammonia be recovered and recycled by pressurizing and cooling exhaust gas containing ammonia, the exhaust gas being exhausted from a production process of a gallium nitride compound semiconductor and then by liquefying ammonia contained in the exhaust gas to recovering liquefied ammonia from the exhaust gas.
This led to propose a method of recovering ammonia gas, including, for example, a dissolution step of dissolving ammonia gas contained in exhaust gas in water, the exhaust gas being exhausted from a process such as a production process of a gallium nitride compound semiconductor; a distillation step of distilling the aqueous ammonia to separate ammonia gas from water; and a liquefaction step of liquefying the separated ammonia gas (JP 2008-7378A). Furthermore, another method was proposed, including cooling and passing exhaust gas containing ammonia through a multitubular adsorber filled with adsorbent for ammonia, the exhaust gas being exhausted from a production process of a gallium nitride compound semiconductor and the like; and after adsorbing ammonia from exhaust gas, desorbing and recovering ammonia under reduced pressure while heating the multitubular adsorber (JP 2000-317246 A).